In general, a separator to be used in zinc/bromine secondary batteries should have the following characteristics:    1) having an ionic conductivity, a low resistivity, and functions of reducing self-discharges with zinc and bromine which occur at both electrode compartments;    2) comprising a membrane having a stability which prevents the bromine dispersion from generating at an anode and does not cause deterioration of the membrane due to the strongly oxidizing bromine;    3) comprising a membrane insusceptible to swelling and flexure and capable of lengthen the life of the batteries;    4) having a lower production cost.
In the current art, it is considered that separators having the aforementioned characteristics which have been put in use include ion-exchange films, fluorinated resin porous films, and polyolefin porous films. Among them, separators made of polyethylenes and fine particulate silica as disclosed in JP-B-5-27233 are cheap and excellent in resistance to oxidation.
However, the separators as disclosed in the JP-B-5-27233 have a thermal resistance problem that the step of attaching an electrode frame to the separator by injection molding causes cracking of the separator due to heating, as described in JP-A-62-17945. Moreover, these separators suffer from cracking of films, i.e., deficiency in resistance to stress-cracking after they have been used for a long period of time as those in zinc/bromine secondary batteries.
In order to overcome these problems, an attempt has been proposed to incorporate ultra-high molecular weight polyethylenes, as disclosed in JP-A-9-231957. However, this art cannot be said sufficiently satisfactory in electrical properties though it could increase the thermal resistance and the resistance to stress-cracking.
Moreover, when polyolefin based fine porous films containing primarily polyolefins and silica are used, the separators have usually insufficiently low bromine permeability to make it impossible to achieve sufficient coulomb (Ah) efficiency. To overcome this difficulty, a technique of rendering the ratio of the number of silicon atoms (Si) to the number of carbon atoms (C) which are exposed on the surfaces including the separator's wall surfaces, i.e., Si/C, not less than 0.2 as determined with an X-ray photoelectron spectroscopy (XPS) has been proposed, as described in JP-A-1-157071.
However, these separators are disadvantageously deteriorated drastically during the use thereof with a great reduction in coulombic efficiency resulting in a short life of the cells, though they have certainly a high initial coulombic efficiency.